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Electrochemical enhancement of solar photocatalysis:Degradation of endocrine disruptor bisphenol-A on Ti/TiO2
films
Zacharias Frontistis a, Vasileia M. Daskalaki a, Alexandros Katsaounis a, Ioannis Poulios b,Dionissios Mantzavinos a,*aDepartment of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greeceb Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
a r t i c l e i n f o
Article history:
Received 9 December 2010
Received in revised form
8 March 2011
Accepted 15 March 2011
Available online 21 March 2011
Keywords:
BPA
Current
Kinetics
Promotion
Titania
Water
* Corresponding author. Tel.: þ302821037797E-mail address: mantzavi@mred.tuc.gr (D
0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.03.030
a b s t r a c t
The photoelectrocatalytic oxidation over immobilized Ti/TiO2 films in the presence of
simulated solar light was investigated for the degradation of bisphenol-A (BPA) in water.
The catalyst, consisting of 75:25 anatase:rutile, was prepared by a sol-gel method and
characterized by cyclic voltammetry, X-ray diffraction and scanning electron microscopy.
Experiments were conducted to assess the effect of applied current (0.02e0.32 mA/cm2),
TiO2 loading (1.3e9.2 mg), BPA concentration (120e820 mg/L), initial solution pH (1 and 7.5)
and the aqueous matrix (pure water and treated effluent) on BPA photoelectrocatalytic
degradation which was monitored by high performance liquid chromatography equipped
with a fluorescence detector. The reaction was favored at anodic currents up to 0.04 mA/
cm2 and lower substrate concentrations, but it was hindered by the presence of residual
organic matter and radical scavengers (e.g. bicarbonates) in treated effluents. Moreover,
a pseudo-first order kinetic model could fit the experimental data well with the apparent
reaction constant taking values between 2.9 and 32.4 10�3/min. The degradation of BPA by
pure photocatalysis or electrochemical oxidation alone was also studied leading to partial
substrate removal. In all cases, the contribution of applied potential to photocatalytic
degradation was synergistic with the photocatalytic efficiency increasing between 24% and
97% possibly due to a more efficient separation and utilization of the photogenerated
charge carriers. The effect of photoelectrocatalysis on the ecotoxic and estrogenic prop-
erties of BPA was also evaluated measuring the bioluminescence inhibition of Vibrio fischeri
and performing the yeast estrogen screening assay, respectively.
ª 2011 Elsevier Ltd. All rights reserved.
1. Introduction surface waters, effluents from wastewater treatment plants,
Bisphenol-A (BPA) is widely used as raw material in the
manufacturing of numerous chemical products, such as
polycarbonate plastics and epoxy resins. Due to its wide-
spread usage, it has been detected in treated drinking water,
; fax: þ302821037852.. Mantzavinos).ier Ltd. All rights reserve
landfill leachates, sediments from lakes, rivers and channels
and tissues of aquatic animals (Garoma et al., 2010). BPA is
thought to be associated with endocrine disruption (Kang
et al., 2007), as well as several other adverse effects
including genotoxicity (Park and Choi, 2009), increasing
d.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 4 2997
cancer cells (Jenkins et al., 2009) and sperm count reduction
(Salian et al., 2009).
In recent years, there have been intensive efforts toward
the development of efficient technologies for the removal of
persistent xenobiotics from aqueous matrices. In this
perspective, advanced oxidation processes are likely to play
a key role and several recent studies have dealt with BPA
degradation by ozonation (Garoma et al., 2010), ultrasound
irradiation (Petrier et al., 2010), dark- (Poerschmann et al.,
2010) and photo-Fenton (Zhan et al., 2006) oxidation, electro-
chemical oxidation (Cui et al., 2009), as well as various hybrid
processes (Torres et al., 2008; Huang and Huang, 2009; Torres-
Palma et al., 2010).
Over the past several years, heterogeneous photocatalysis
has received enormous attention for the treatment of various
classes of organic contaminants found in waters and waste-
waters. Of the various semiconductors, TiO2 has almost
exclusively been employed in environmental applications and
the fundamentals of the process can be summarized as
follows (Malato et al., 2009): the electronic structure of TiO2,
consisting of an empty conduction band and a filled valence
band, facilitates the formation of electron/hole pairs when the
semiconductor absorbs photonic energy greater than its band
gap energy of about 3.2 eV, i.e. at wavelengths below about
390 nm. Holes are strong oxidizing agents and electrons are
good reducing agents, therefore both promote redox reac-
tions. Most organic photodegradation reactions utilize the
oxidizing power of holes either directly or indirectly, i.e.
through the formation of hydroxyl radicals and other reactive
oxygen species.
In recent studies, BPA degradation has been investigated
bymeans of UVA (Tsai et al., 2009; Guo et al., 2010) and natural
solar irradiation (Rodriguez et al., 2010) over TiO2 suspensions
with emphasis given on the effect of various operating
conditions on performance. Although solar photocatalysis is
an attractive and sustainable water treatment technology, it
suffers a major drawback, i.e. commercially available, pure
TiO2 can only be activated by UVA light which comprises just
3e5% of the solar spectrum. Enhanced catalytic activity for the
degradation of BPA under visible light was achieved doping
either pure TiO2 (Subagio et al., 2010) or supported on acti-
vated carbon (Yap et al., 2010) with nitrogen, co-doping TiO2
with nitrogen and carbon (Wang and Lim, 2010) and adding
polyethylene glycol (Kuo et al., 2010) in an attempt to narrow
the band gap of titania and/or change its surface properties.
All the aforementioned studies (Tsai et al., 2009; Guo et al.,
2010; Kuo et al., 2010; Rodriguez et al., 2010; Subagio et al.,
2010; Wang and Lim, 2010; Yap et al., 2010) refer to slurry
systems that suffer a serious drawback, i.e. the need to
separate the fine catalyst particles from the treated solution.
This can be overcome immobilizing the catalyst on suitable
supports or carriers; Wang et al. (2009) studied BPA degrada-
tion by UVC irradiation over TiO2 fixed to polyurethane foam
cubes, while Nakashima et al. (2002) immobilized TiO2 on PTFE
mesh sheets to degrade BPA and other endocrine disruptors
under UVA irradiation. Nonetheless, the quantum efficiency
of immobilized catalyst is generally lower than that of the
suspended one due to increased mass transfer limitations of
the contaminant to the catalyst and a decrease of its surface
area (Philippidis et al., 2010).
Away to improve the performance of immobilized TiO2 is to
fix the catalyst on a conductive substrate and apply an external
voltage in a photoelectrochemical cell, thus driving the photo-
generated electrons to the cathode and, consequently, mini-
mizing the rate of electron/hole recombination. The process,
which isalsoreferredtoasphotoelectrocatalysis (PEC),hasbeen
recently demonstrated for the UVA-induced degradation of
aniline (Ku et al., 2010) and the inactivation of Escherichia coli
colonies (Philippidis et al., 2010) onTi/TiO2 electrodes at applied
potential values up to 2 V vs Ag/AgCl, the solar light-induced
degradation of rhodamine B on nitrogen-doped Ti/TiO2 elec-
trodes up to 1.4 V vs saturated calomel electrode (SCE) (Han
et al., 2010) and the solar light-induced degradation of phenol
on Si/TiO2 arrays at 3 V vs SCE (Yu et al., 2009).
ThedegradationofBPAbyPEChasonlymerelybeenreported
in the literature. Li et al. (2005) prepared Au/TiO2/(IneSn oxide)
filmsandassessed the contributionof gold to thedegradationof
16mg/L BPA under UVA irradiation and up to 8 V vs SCE applied
potential; the same research group (Xie and Li, 2006) also tested
Au/TiO2/Tifilmsfor thedegradationof11.2mg/LBPAunderUVA
orvisible irradiationandupto1.5mAappliedcurrent. Inarecent
study, Brugnera et al. (2010) synthesized TiO2 nanotubular
arrays for the degradation of 22.8 mg/L BPA under UVA irradia-
tion and up to 1.5 V vs Ag/AgCl potential.
The scope of this work was to investigate BPA degradation
by solar photoelectrocatalysis on Ti/TiO2 films and compare
its efficiency with photocatalysis (PC) and electrochemical
oxidation (EO). The effect of various operating conditions such
as applied current, BPA concentration, catalyst loading, solu-
tion pH and the water matrix on kinetics has been examined.
2. Materials and methods
2.1. Catalyst preparation
Immobilized TiO2 films on Ti substrates were prepared by
a sol-gel procedure, as follows: 4.2 g of the non-ionic surfac-
tant Triton X-100 (polyoxyethylene-(10) isooctylphenyl ether)
slurry were mixed with 22.8 mL of ethanol, followed by the
addition of 3.9mL of glacial acetic acid and 2.16mL of titanium
isopropoxide under vigorous stirring. Self organization of the
surfactant in this original sol creates organized assemblies
which act as templates defining nanoparticle size. The
surfactant was burned out during calcination. After stirring
for a few minutes, a squared shaped titanium substrate
(2.4� 2.4 cm)was dipped in the above solution andwithdrawn
at a constant speed. The nanocomposite film formed by
dipping was left to dry in air for a fewminutes and then it was
calcined in an oven at 550 �C for 10 min (Bouras and Lianos,
2005). When the film was taken out of the oven it was trans-
parent and optically uniform. The above procedure was
repeated until a certain amount of titania in the range
1.3e9.2 mg (the selection was based on preliminary tests,
previous experience and technical constraints) was deposited
on the Ti substrate; the latter was first sandblasted to ensure
good adhesion of the deposit on its surface followed by
a chemical treatment (using a 1 M oxalic acid solution at the
boiling point for 60 min) in order to obtain a totally clean
surface.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 42998
2.2. Experimental procedure
Photocatalytic experiments were performed using a solar
simulator (Oriel, model 96000) equipped with a 150 W xenon,
ozone free lamp in an open cylindrical pyrex cell at ambient
conditions under continuous stirring. Current in the range
0.02e0.32 mA/cm2 was applied using a galvanostatepotentio-
stat (Amel Instruments, model 2053) with the Ti/TiO2 film
operatingas theanode,whilea zirconiumplatewasusedas the
cathode. The experimental setup is illustrated in Scheme 1.
In a typical experiment, the appropriate amount of BPA
was dissolved in ultrapure water (UPW) (EASYpureRF e
Barnstead/Thermolyne, USA) and the resulting solution (in
the range 120e820 mg/L BPA) was added 5 mM HClO4 as the
supporting electrolyte; 60 mL were then introduced in the
reaction vessel and left for 20 min in the dark to equilibrate
prior to applying solar irradiation and/or current. To assess
the effect of water matrix, experiments were also conducted
with BPA spiked in the treated effluent (TE) of the activated
sludge process of the municipal wastewater treatment plant
of Chania, W. Crete, Greece. The effluent had 8.4 mg/L of
dissolved organic carbon (DOC), while its pH and conductivity
were about 8 and 0.81mS/cm, respectively. The concentration
of chlorides, sulfates, nitrates, nitrites and bicarbonates was
222.1, 60.3, 25.9, 57.1, and 182.1 mg/L, respectively.
2.3. Analytical techniques
Changes in BPA concentration were followed by high perfor-
mance liquid chromatography (HPLC, Waters 2690) equipped
with a Luna 54 (18C2) 100 A column and two detectors con-
nected in series, namely a diode array detector (Waters 996)
and a fluorescence detector (Waters 474). The mobile phase
Scheme 1 e Experimental setup.
consisted of 65:35 acetonitrile:water at a flow rate of 1mL/min
and ambient temperature. BPA was monitored by the fluo-
rescence detector, while the diode array set at 280 nm was
used to identify possible reaction by-products.
DOC was measured by NDIR gas analysis on a Shimadzu
5050A TOC analyzer.
The catalyst was characterized for its phase composition
bymeans of X-ray diffraction (XRD) using a Philips, PW1830/40
powder diffractometer. Scanning electron microscopy (SEM)
was performed using a JEOL JMS-6300-F microscope.
The electrochemical/photoelectrochemical characterization
of theTiO2filmswascarriedout inasingle-compartment, three-
electrodecelldescribed indetail elsewhere (Chatzisymeonetal.,
2010) in conjunction with the solar simulator. The electrolyte
volume in the compartmentwas 40mL. A platinumwire served
as thecounterelectrode,whileHg/Hg2SO4/K2SO4 (MSE) (Ref-621,
Radiometer Analytical) was employed as the reference elec-
trode. Voltammograms were run for at least 3 times since
preliminary experiments showed that steady state conditions
could be reached after the third run.
2.4. Ecotoxicity and estrogenicity assays
The luminescent marine bacteria Vibrio fischeri were used to
assess the acute ecotoxicity of BPA prior to and after PEC
treatment. The inhibition of bioluminescence of V. fischeri
exposed to undiluted BPA solutions for 15 min was measured
using a LUMIStox analyzer (Dr Lange, Germany) and the
results were compared to an aqueous control.
The yeast estrogen screening (YES) bioassay was employed
to assess the effect of PEC on estrogenicity according to the
protocol developed byRoutledge and Sumpter (1996)with some
modifications. In brief, standard solutions of estrogen 17b-
estradiol and sample extracts were produced in ethanol and
10mLofdilutionseriesweredispensed into triplicatewellsof96-
well microtiter plates. After evaporation to dryness at room
temperature, 0.2 mL of growth medium containing chlor-
ophenol red-b-D-galactopyranoside and the yeast cells were
added, followed by incubation at 32 �C for 72 h. The absorbance
of the medium was measured using a microplate reader (LT-
4000MSMicroplate Reader, Labtech). The absorbance at 540 nm
was regarded as estrogenic activity after subtraction of absor-
bance at 640 nm to correct for yeast growth.
3. Results and discussion
3.1. Catalyst characterization
XRD patterns were taken in the range of 2q between 20� and
70� with a scanning rate of 0.05�/s (Fig. 1). As seen, TiO2 films
can be successfully developed on Ti substrate containing
peaks that are attributed to both the anatase and rutile pha-
ses. Phase composition was estimated by the integral inten-
sities of anatase (101) and rutile (110) reflections (Cullity, 1978)
and it was found to be about 75% and 25%, respectively. The
primary crystallite size of TiO2 was estimated at 25 � 3 mm
applying the Scherrer’s equation (Cullity, 1978). On the other
hand, SEM images (Fig. 1) reveal that the Ti surface is highly
porous due to its sandblasting and subsequent chemical
Fig. 1 e XRD patterns (top) and SEM images (bottom) of Ti/TiO2 film and Ti support.
-5
0
5
10
15
20
25
-0.5 0 0.5 1 1.5 2 2.5 3E, V vs MSE
I, µ
A/c
m2
Dark
Solar
Fig. 2 e Linear sweep voltammograms of Ti/TiO2 in the
dark and under solar illumination. Sweep rate [ 50 mV/s;
pHo [ 1.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 4 2999
treatment; this allows the formation of a continuous network
of TiO2 layers with large surface area, where more catalyst
active centers are available for redox reactions to occur.
3.2. Photoelectrochemical characterization
As most applications of photoelectrochemical systems
involve the transfer of electrons across the solid/electrolyte
interface, current density-applied potential recording tech-
niques are commonly used for their characterization. Linear
sweep voltammograms of Ti/TiO2 electrode in 0.1 M HClO4 in
the dark and under solar illumination, recorded between �0.4
andþ3 V vs MSE at a sweep rate of 50 mV/s, are given in Fig. 2.
The low dark current density between�0.4 and 1.8 V is typical
of the n-type semiconductive behavior of the synthesized TiO2
specimens, while the observed increase at potentials greater
than þ1.8 V vs MSE can be attributed to water decomposition
and oxygen evolution. At potentials more negative than
�0.4 V, both a cathodic peak and an ill-defined anodic humpor
peak occur (not shown), with the former corresponding to the
reduction of surface Ti(IV) species and the latter to the re-
oxidation of surface Ti(III) species.
On the contrary, upon illumination a significant increase in
the anodic current density above �0.4 V vs MSE occurred, as
a result of the photogenerated holes. According to the
literature (Morrison, 1980; Finklea, 1988; Memming, 2001),
illumination of a semiconductoreelectrolyte interface with
light energy greater than its band gap energy generates elec-
tronehole pairs at the electrode surface. The simultaneous
application of a bias positive to the flat-band potential
produces a bending of the conduction and valence bands
0
0.2
0.4
0.6
0.8
1
C/C
o
PECPCEO
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 43000
which, in turn, causes a more effective separation of the
photogenerated carriers within the space charge layer; this
increases the photocurrent that begins to flow, thus
promoting the oxidative degradation process. The potential
gradient forces the electrons toward the counter electrode,
thus leaving the photogenerated holes to react with H2O and/
or OH� to yield hydroxyl radicals or to attack directly the
organics present in the solution, i.e.:
Anode (working electrode):
TiO2 þ hn/TiO2 � e�cb þ TiO2 � hþ
vb (1)
0 30 60 90 120 150 180time, min
Fig. 3 e Variation of normalized BPA concentration during
PEC, PC and EO. Conditions: [BPA]o [ 300 mg/L in UPW;
I [ 0.08 mA/cm2; pHo [ 1; TiO2 loading [ 2.6 mg.
TiO2 � hþvb þH2Os/TiO2 �OHs$þHþ (2)
TiO2 � hþvb þOH�
s /TiO2 �OHs$ (3)
TiO2 � e�cb þ TiO2 � hþ
vb/recombination (4)
Cathode (counter electrode):
2H2O þ 2e� / H2 þ 2OH� (5)
where the subscripts cb and vb denote the conduction and
valence bands, respectively and hþ and e� denote the photo-
generated holes and electrons, respectively.
3.3. Efficiency of PEC in relation to PC and EO
Fig. 3 shows normalized BPA concentrationetime profiles
during PEC, PC and EO treatments at 300 mg/L initial concen-
tration and pHo ¼ 1. PC oxidation occurs slowly yielding only
30% conversion after 180 min of reaction; nonetheless,
conversion becomes quantitative when 0.08 mA/cm2 of
current is applied. The enhanced PEC performance may only
partly be attributed to electrochemical degradation reactions
since EO alone results in about 10% conversion. The applica-
tion of a positive current producing a potential greater than
the flat-band potential of the Ti/TiO2 electrode decreases the
charge recombination process (eq. (4)), which is essential for
promoting substrate degradation.
3.4. Effect of applied current
Fig. 4 shows the effect of varying current in the range
0.02e0.32 mA/cm2 on BPA degradation by PEC and EO treat-
ments at 300 mg/L initial concentration and pHo ¼ 1. There
appears to be a threshold current at about 0.02 mA/cm2 below
which the promotion of PC is not evident, i.e. 37% and 31% BPA
conversion was recorded after 180 min at 0.02 and 0 mA/cm2,
respectively. Increasing current from 0.02 to 0.04 mA/cm2 had
a pronounced effect on PC leading to complete BPA removal
after 120e180 min, while applying higher currents did not
practically improve degradation (e.g. in terms of final
conversion).
Unlike PEC, increasing current from 0.02 to 0.16 mA/cm2
enhanced EO performance, i.e. the final BPA conversion was
5%, 18% and 24% at 0.02, 0.04 and 0.16 mA/cm2, respectively
(Fig. 4b). Notably, operation at 0.32 mA/cm2 led to decreased
performance and this may be attributed to the fact that the
electrode partially disintegrated loosing 25% of its mass after
180 min of reaction. This phenomenon did not occur at lower
currents as evidenced weighing the electrodes before and
after the reaction. Conversely, PEC at 0.32 mA/cm2 was not
affected by the electrode’s partial disintegration and this may
be due to the fact that (i) the remaining electrode is still
active enough to induce fast BPA degradation (see Section
3.5), and/or (ii) the leached TiO2 (0.67 mg) contributes signif-
icantly to the PC reaction rate, thus compensating any
activity loss due to dissolution. This was verified in prelimi-
nary PC runs with 0.65 mg of Degussa, P-25 TiO2 either
immobilized on Ti or suspended in the reaction mixture; the
immobilized catalyst gave far lower conversions than the
respective slurry system.
Previous studies (Li et al., 2005; Brugnera et al., 2010) have
shown that BPA degradation by PEC can be described by
pseudo-first order kinetics, i.e.:
�dCdt
¼ kPECC5lnCo
C¼ kPECt (6)
where kPEC is an apparent kinetic constant. If the results of
Fig. 4a are plotted in the form of eq. (6), straight lines passing
through the origin (not shown) fit the experimental data very
well. From the slopes of the resulting lines, kPEC values can
be computed and they are summarized in Table 1 alongside
the coefficients of linear regression; as seen, kPEC at 0.02 mA/
cm2 is an order of magnitude lower than at 0.04e0.16 mA/
cm2.
Likewise, data from the respective PC experiments were
also fitted to pseudo-first order kinetics (not shown) from
which the respective kPC constants were computed; these,
alongside kPEC values, were employed to quantify the extent of
photoelectrochemical enhancement (E ), as follows:
E ¼ kPEC � kPC
kPEC(7)
As also seen in Table 1, the extent of enhancement is only
about 24% at 0.02 mA/cm2 but it increases to 90.5 � 1.2% at
currents between 0.04 and 0.16 mA/cm2.
Table 1 e Apparent kinetic constants (eq. (6)) and extentof enhancement (eq. (7)) associated with PEC degradationof BPA at various conditions. Numbers in brackets showcoefficients (r2) of linear regression (eq. (6)). ND: notdetermined because PC run was not performed.
Co,mg/L
I,mA/cm2
pHo Matrix Catalystloading, mg
kPEC, 10�3/
minE,%
300 0.02 1 UPW 2.6 2.9 (0.930) 24.1
300 0.04 1 UPW 2.6 20.8 (0.999) 89.4
300 0.08 1 UPW 2.6 20.6 (0.974) 89.3
300 0.16 1 UPW 2.6 26.4 (0.983) 91.7
300 0.16 1 UPW 9.2 23.6 (0.996) 72
300 0.16 1 UPW 1.3 20.5 (0.998) 90.2
120 0.16 1 UPW 2.6 32.4 (0.987) 97.2
500 0.16 1 UPW 2.6 19.5 (0.988) 89.2
820 0.16 1 UPW 2.6 11.2 (0.999) ND
300 0.16 7.5 UPW 2.6 22.8 (0.998) 53.1
300 0.16 8 TE 2.6 10.1 (0.977) 85.1
0
0.2
0.4
0.6
0.8
1
0 30 60 90 120 150 180time, min
C/C
o
PC-2.6mgPC-9.2mgPEC-2.6mgPEC-9.2mgPC-1.3mgPEC-1.3mg
Fig. 5 e Variation of normalized BPA concentration during
PEC and PC as a function of catalyst loading. Conditions:
[BPA]o [ 300 mg/L in UPW; pHo [ 1; I [ 0.16 mA/cm2.
0
0.2
0.4
0.6
0.8
1
0 30 60 90 120 150 180time, min
C/C
o 0 mA/cm20.02 mA/cm20.04 mA/cm20.16 mA/cm20.32 mA/cm2
a
0
0.2
0.4
0.6
0.8
1
0 30 60 90 120 150 180time, min
C/C
o
0.02 mA/cm2
0.04 mA/cm2
0.16 mA/cm2
0.32 mA/cm2
b
Fig. 4 e Variation of normalized BPA concentration during
(a) PEC and (b) EO as a function of applied current.
Conditions: [BPA]o [ 300 mg/L in UPW; pHo [ 1; TiO2
loading [ 2.6 mg.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 4 3001
3.5. Effect of catalyst loading
Fig. 5 shows the effect of increasing catalyst loading from 1.3
to 9.2 mg on BPA degradation by PEC and PC treatments at
300 mg/L initial concentration and pHo ¼ 1. An increase in
catalyst loading had practically little effect on PEC degradation
with kPEC being 23.5� 3 10�3/min at any loading. Nevertheless,
the extent of enhancement decreased from 91 � 0.8% at
1.3e2.6 mg to 72% at 9.2 mg; this is due to the fact that the
respective kPC value, which remained unchanged between 1.3
and 2.6 mg, increased by as much as three times between 2.6
and 9.2 mg catalyst.
In an immobilized PC system, the reactant diffuses from
the bulk solution through a boundary layer to reach the elec-
trode double layer, where it gets adsorbed onto the active sites
of the catalyst and reacts. The optimum film thickness
depends on the light penetration depth and the width of the
space charge layer. At reduced catalyst loadings, fewer surface
sites are available for reactions including the generation of
fewer holes and electrons, as well as secondary oxidizing
species. In the case of PEC, the efficient separation of charge
carriers and, consequently, their availability to react for longer
periods seems to be critical, thus compensating the reduced
rate of generation at lower loadings.
3.6. Effect of BPA concentration
The effect of initial BPA concentration on PEC degradationwas
also investigated at 0.16 mA/cm2, 2.6 mg catalyst loading and
pHo ¼ 1 and the results are shown in Fig. 6 and Table 1. As
seen, increasing BPA concentration resulted in decreased
kinetics; for example, kPEC was 32.4, 26.4, 19.5 and 11.2 10�3/
min at 120, 300, 500 and 820 mg/L, respectively. At a constant
catalyst loading, performance will be dictated by the catalyst
sites to substrate molecules ratio. The fact that degradation
decreases at higher initial BPA concentrations may be asso-
ciated with the availability of fewer active sites, thus trig-
gering a competitive adsorption between BPA and reaction by-
products (whose determination was not feasible with the
analytical protocols employed in this study) onto the catalyst
surface which thereby decreases the concentration of
oxidizing species attacking BPA (Jain and Shrivastava, 2008).
0
150
300
450
600
750
0 30 60 90 120 150 180time, min
BP
A c
on
ce
ntra
tio
n, µ
g/L
120µg/L
300µg/L
500µg/L
820µg/L
Fig. 6 e Variation of BPA concentration in UPW during PEC
as a function of initial concentration. Conditions:
I [ 0.16 mA/cm2; pHo [ 1; TiO2 loading [ 2.6 mg.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 43002
3.7. Effect of pH and the water matrix
In further studies, BPA degradation in UPW was studied at
pHo ¼ 7.5 and the results are shown in Fig. 7. Comparing the
performance at acidic and near-neutral conditions (Figs. 4 and
7 and Table 1), it is evident that PEC degradation was nearly
equally fast at either conditions with kPEC being 26.4 and
22.8 10�3/min at pHo 1 and 7.5, respectively. (It should be
pointedouthere that althoughpHwas left uncontrolled during
the reactions it did not change more than 0.1e0.2 units.)
Interestingly and unlike PEC, the individual processes were
favored at pHo ¼ 7.5 with the final conversion being 85% and
63% for PC and EO, respectively, while the corresponding
values at acidic conditions were only 31% and 24%. In terms of
kinetics, kPC increased by asmuch as about 5 times going from
acidic to near-neutral conditions and this explains the
substantial drop of process enhancement from 91.7% to 53.1%.
At thepHvalues under consideration, BPA is inmolecular form
in the solution since its pKa value is 9.6e10.2 (Bautista-Toledo
et al., 2005). In this respect, the pH effect cannot be explained
0
0.2
0.4
0.6
0.8
1
0 30 60 90 120 150 180time, min
C/C
o
PC/TEEO/TEPEC/TEPC/UPWEO/UPWPEC/UPW
Fig. 7 e Variation of normalized BPA concentration during
PEC, PC and EO in UPW at pHo [ 7.5 and TE at pHo [ 8.
Conditions: [BPA]o [ 300 mg/L; I [ 0.16 mA/cm2; TiO2
loading [ 2.6 mg.
on the basis of electrostatic interactions between the ampho-
teric catalyst surface (which would be positively charged at
pHo¼ 1 andnegatively chargedat pHo¼ 7.5) and theuncharged
BPA molecules. The reduced performance at pHo ¼ 1 could
partially be due to the fact that the formation of hydroxyl
radicals is suppressed at lower pH values (Hapeshi et al., 2010).
In a final set of experiments, process performance was
evaluated in a secondary treated effluent at inherent condi-
tions (i.e. pHo¼ 8 and 0.81mS/cmconductivity) and the results
are also shown in Fig. 7. The presence of 8.4 mg/L of organic
carbon (this is 35 times the carbon contained in 300 mg/L BPA)
impeded degradation and this was more pronounced for PEC
and PC. As seen in Table 1, kPEC in TE decreased by about 50% in
relation to UPW, while the respective reduction for kPC was
7-fold. Nevertheless, the PEC degradation of BPA became
quantitative after 180minof reaction. Photogenerated reactive
species are partly consumed to attack effluent organic matter
(this is consistent with the fact that reaction rates decrease
with increasing carbon concentration) and this explains the
reduced performance in TE samples compared to UPW
samples. Furthermore, the presence of inorganics like bicar-
bonates that act as radical scavengersmay, to some degree, be
responsible for decreased conversions in TE.
3.8. Removal of ecotoxicity and estrogenicity
The effect of PEC on the ecotoxic and estrogenic properties of
BPA is shown in Fig. 8. BPA is only slightly toxic to marine
bacteria with 38% inhibition of bioluminescence being recor-
ded at 300 mg/L; this value decreased to 26% after 120 min of
reaction, which corresponds to quantitative BPA removal
(Fig. 7). In parallel, BPA exhibits 5 mg/L of equivalent estro-
genicity and this is reduced by 25% after 60 min; these results
imply that PEC degradation by-products are less ecotoxic and
estrogenic than BPA.
It should be noted here that identification of likely reaction
by-products was not feasible with the analytical tools and
protocols employed in thiswork. Nonetheless, an attemptwas
made to perform a carbon balance in the liquid phase
following BPA and DOC profiles during PEC degradation in
0
10
20
30
40
50
0 30 60 90 120time, min
In
hib
itio
n, %
0
1
2
3
4
5V. fischeriEstrogenicity E
qu
iv
ale
nt e
stro
ge
nic
ity
, µ
g/L
Fig. 8 e Variation of inhibition to V. fischeri (left axis) and
estrogenicity (right axis) during PEC in UPW at pHo [ 7.5.
Conditions: [BPA]o [ 300 mg/L; I [ 0.16 mA/cm2; TiO2
loading [ 2.6 mg.
0
800
1600
2400
3200
4000
0 30 60 90 120time, min
Ca
rb
on
c
on
ce
ntra
tio
n, µ
g/L
DOC
BPA
Fig. 9 e Organic carbon balance during PEC of 5 mg/L BPA in
UPW. Conditions: I [ 0.16 mA/cm2; TiO2 loading [ 2.6 mg;
pHo [ 7.5.
wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 2 9 9 6e3 0 0 4 3003
UPW; the experiment was carried out at an elevated BPA
concentration of 5mg/L (this corresponds to 4mg/L of DOC) to
allow for reliable DOC analysis. As clearly seen in Fig. 9, most
of the remaining organic carbon in the liquid phase is due to
unreacted BPA; for instance, the carbon contained in BPA
accounts for 82% of DOC after 120 min of reaction, where 53%
of BPA has been removed. This implies that most of the by-
products do not accumulate in the liquid but they are rapidly
mineralized to carbon dioxide and water.
4. Conclusions
The degradation of bisphenol-A, an emerging aqueous phase
pollutant, has been investigated by means of solar irradiation
over fixed Ti/TiO2 catalysts enhanced by the simultaneous
application of an electric field. The major conclusions drawn
from this study are summarized as follows:
(1) The proposed process is advantageous since (i) it utilizes
renewable energy, thus promoting sustainability, and (ii)
catalyst immobilization enables its easy recovery at the
end of the treatment.
(2) The concurrent use of two or more processes to improve
performance is conceptually attractive if the overall effect
is synergistic rather than additive. This seems to be the
case since the application of relatively low currents (i.e. up
to 0.04e0.08 mA/cm2) promotes dramatically the rate of
the respective photocatalytic process with the level of
enhancement reaching values up to 97%.
(3) Promotion of photocatalysis is thought to be associated
with the efficient separation of photogenerated holes from
electrons, thus maximizing their utilization as primary
oxidants or source to generate secondary oxidants like
hydroxyl (and other) radicals.
(4) Performance can be affected by parameters like applied
current, catalyst loading, solution pH and the composition
of the water matrix; the presence of dissolved species may
partly impede the removal of BPA from secondary treated
effluents.
Acknowledgments
The authors wish to thank Dr D. Fatta-Kassinos (University of
Cyprus) and Dr E. Routledge (Brunel University, UK) for
supplying yeasts for the YES test.
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